Cardiovascular disease is the leading cause of death in the U.S., and will be the primary cause of mortality in developing countries by 2010, as estimated by the WHO. Nevertheless, the demand for transplantation exceeds the availability of donor hearts. In this regard, cardiac regeneration has recently become an active area of research. Over the past few years, numerous reports demonstrate cardiac progenitors from diverse fetal and adult tissues outside the cardiovascular system, including adipose tissues, amniotic fluid, bone marrow, placenta, skeletal muscle, and testes (Franco et al., 2007). However, their low frequency of cardiac differentiation (Murry et al., 2004) and lack of long-term benefits fail to achieve cardiac cell regeneration (Fazel et al., 2006, 2008). Bone marrow cells, for instance, may improve the function of the infarcted heart mainly by promoting angiogenesis or cell survival without cardiac muscle regeneration (Fazel et al., 2006, 2007). A recently discovered cardiac progenitor population marked by the expression of the LIM homeodomain transcription factor isl1 (Laugwitz et al., 2005; Moretti et al., 2006; Qyang et al., 2007) is an attractive target to study cardiac regeneration. A multipotent islet 1 (isl1+) cardiovascular progenitor (MICP) is able to give rise to the major three cell types of the heart: cardiomyocytes, smooth muscles and endothelial cells, and has clonogenic and self-renewing ability (Laugwitz et al., 2005; Moretti et al., 2006). In Isl1 knockout mice, histological analysis of mutant hearts between embryonic day (ED) 9.0 and ED9.5 showed a misshapen single heart ventricle as the cause of death (Cai et al., 2003). Lineage tracing studies in mice document that isl1+ progenitors give rise to over two thirds of the cells in the heart, mostly on the right side, including most of the conduction system: the sinoatrial (SA) node, the atrioventricular (AV) node, His-bundle, and Purkinje fiber complex (Cai et al, 2003; Laugwitz et al., 2005; Moretti et al., 2006; Sun et al., 2007). Disruption of development, differentiation or maturation of any of these components can lead to arrhythmias such as sinus arrest, AV block, ventricular tachycardia and sudden death (Bruneau et al., 2001).
The generation and expansion of diverse cardiovascular cell lineages is a critical step during human cardiogenesis with major implications for congenital heart disease. Unraveling the mechanisms for human heart cell lineage diversification has been hampered by the lack of genetic tools to purify early cardiac progenitors and define their developmental potential1-4. Recent studies in the mouse embryo have identified a multipotent cardiac progenitor (MICP), which contributes to all the major cell types in the murine heart5-8. In contrast to murine development, human cardiogenesis has a much longer onset of heart cell lineage diversification and expansion, suggesting divergent pathways.
Accordingly, improved methods for identifying human pluripotent progenitors which give rise to human multipotent cardiac progenitors (hMICP) for the development of human diverse cardiovascular cell lineages and the production of human cardiac tissue are needed.